Marine propulsion systems, such as marine outboard engines, typically include a gear case assembly for transmitting torque from the engine to the propeller or impeller of the propulsion system. The gear case assembly includes a gear case housing various components, such as shafts, gears and the like, that are used to achieve this torque transmission. At least some of these components, which may be housed in different interconnected chambers and passages formed within the gear case, are immersed in lubricant. During operation of the propulsion system, the lubricant is circulated through various chambers and passages of the gear case and around the various components. The operation of the engine causes heating of the lubricant in the gear case. The heated lubricant expands, occupying more space than the cold lubricant. In a sealed gear case filled with oil, this would raise the pressure therein significantly. It is therefore common to provide a certain amount of air within the gear case that can be compressed so as to compensate for the expansion of the lubricant and to prevent the pressure within the gear case from reaching an unacceptably high level.
Most gear case assemblies have sufficiently large volumes and a sufficient amount of compressible air therein to accommodate the expansion of the lubricant. The expansion of the lubricant during operation of the engine can however be a problem in some gear case assemblies where the overall volume of the chambers and passages is relatively small. One example of such a gear case assembly where the overall volume of the chambers and passages is relatively small is a gear case assembly which includes an electric actuator for controlling the shift between forward and reverse operation of the propeller. The electric actuator is housed inside the gear case but isolated from the chambers and passages in which lubricant is present, thus reducing the volume that could otherwise be available for lubricant and air.
There is therefore a need for an outboard engine and a gear case assembly that can accommodate a sufficient amount of lubricant as well as expansion of the lubricant without also increasing the overall volume of the gear case or the propulsion system.